A diffuser is a device used to increase static pressure of a working fluid and it does so by decreasing the fluid kinetic energy. The amount of energy thus saved is, through the increase in pressure, able to do work elsewhere. For example, in a gas turbine an increase in pressure ratio across a turbine section will result in more work done by the turbine.
An increase in static pressure in an exhaust duct may be achieved simply by progressively increasing the cross-sectional area of the duct in the fluid flow direction, expanding the fluid. However, the flow behaviour of such expanded fluid is dependent on several factors and not easily predicted to the extent there is no recognised best method of establishing an optimum diffuser profile for a given situation. For a given turbine, different engineers are likely to come up with different diffuser profile designs, each having different performance characteristics.
One factor generally regarded of importance is boundary layer separation. When the fluid next to a diffuser wall (the boundary layer) becomes turbulent and separates from the wall there is a loss in diffusing area and pressure recovery is reduced, i.e. the diffuser performance is degraded. Effectively, wherever the flow separates in a diffuser the static pressure (and hence recovery) is fixed at that point, i.e. diffuser exhaust pressure equals static pressure at the separation point. It is well known that wider angles of divergence in a diffuser encourage flow separation whereas smaller angles do not.
A conservative approach to the flow separation problem will generally result in a diffuser of small divergence angle and relatively long length. However, a long diffuser has disadvantage in that where it is part of a gas turbine engine it makes the engine design more complicated and expensive. Also, the length of an engine may be of significant importance for example on an oil platform at sea where available space is limited.
The opposite, aggressive approach, to diffuser design may provide a greater divergence angle and be shorter in length but may suffer in terms of best performance.